1. Field of the Invention
The present invention relates to an indicating instrument which uses a stepping motor as a primary drive of a pointer.
2. Description of the Related Art
In an indicating instrument in which a stepping motor is used as a primary drive of a pointer, a reference position of the pointer is set by rotating the pointer toward a zero-reset position and stopping the pointer at the zero-reset position with a stopper mechanism, in order to carry out a step-out reset.
Induced voltage is generated in a field winding during when the pointer moves, i.e., when a magnet rotor of the stepping motor rotates. The induced voltage will not be generated in the field winding if the pointer stops at the zero-reset position and the rotation of the magnet rotor is stopped.
Given this factor, the field winding of the stepping motor is switched to a non-excited state during the course of driving the stepping motor with alternating current zero-reset voltage, and it is judged that the pointer has reached the zero-reset position when the induced voltage generated in the field winding becomes lower than voltage of a predetermined threshold value (for reference, see JP-A 2001-314099 and 2002-250641).
Conventionally, a switch circuit for switching the field winding to the non-excited state is provided to detect the induced voltage. In addition, in order to carry out zero-reset judgment (judgment that the pointer is in contact with the stopper mechanism) on the basis of the induced voltage by a CPU or the like, the induced voltage is loaded with a predetermined sampling period through an A/D converter, the loaded data of induced voltage is processed with an averaging process or the like, and thereafter, the threshold value is judged. Therefore, there have been problems that the number of circuit component is large and control becomes complicated.